To prevent adhesion and sticking between substrates such as paper or plastic and pressure-sensitive bonding materials, release properties have hitherto been imparted to the surface of the substrate by forming thereon a cured coat of a silicone composition. Addition reaction-based methods of forming release coats are widely used to form silicone cured coats on such substrate surfaces because the curability is excellent and such methods are able to address a variety of release property requirements, from low-speed release to high-speed release. Condensation reaction-based methods of forming release coats, which saw practical use earlier than addition reaction-based methods, are still in use today because they have the advantage of not being affected by addition reaction catalyst poisons.
These reaction curing-based release coat-forming methods make use of materials in which the silicone composition is dissolved in an organic solvent, types of materials in which an emulsifying agent is used to disperse the silicone composition in water and render it into an emulsion, and solventless-type materials that consist only of silicone. Because solvent-type materials are harmful to the human body and the environment, owing to safety concerns, a transition from these to solventless-type materials or emulsion-type materials is currently underway. Emulsion-type materials, in addition to their safety, can be freely diluted with water and used, and can also be freely mixed with other aqueous materials and emulsion materials. For these and other reasons, emulsion-type materials are outstanding in terms of applications and are expected to see increased use in the future.
However, it has been noted to date that, compared with solvent-type compositions, emulsion-type compositions, following application to a substrate and curing, have an to inferior cured coat appearance and surface smoothness. One cause given for this is the water that is used as the silicone-dispersing medium. The reason is thought to be that water, compared with other solvents, has a surface tension which is too high, lowering the wettability and leveling ability of the emulsion composition on the substrate surface. This influence is especially striking in cases where the release composition is applied onto a film substrate having good surface smoothness and low surface tension. Therefore, when producing a release film by applying a release composition onto a commercial film, use is generally made of a solvent-type release composition.
The processing technique whereby a release composition is applied onto a film during production on a film production line, simultaneously applies to the film a release composition is known as in-line coating; emulsion-type compositions can be used for such coating. Emulsion-type compositions are chosen because of such constraints on film production lines as the inability to use flammable liquids and other dangerous materials and the difficulty of installing bulky coating equipment along the line where space is tight. Even at in-line coating, the low wettability and leveling ability of the emulsion-type composition on the substrate surface diminishes the appearance and surface smoothness of the release film. Hence, there remains a desire today for further improvement in the appearance and surface smoothness of release films.
The general trend of late in the solvent use environment has been toward increasing severity, as evidenced by, for example, reductions in environment impacts, stronger restrictions on solvent emissions, improved safety in the work environment, and the rising costs associated with the storage and handling of dangerous materials. Given the push today to eliminate as much as possible residual solvents from materials used close at hand in daily life, such as food packaging, cooking materials, children's commodities and indoor building materials, the range of potential applications for solventless emulsion compositions is growing.
Among conventional methods for improving the appearance and surface smoothness of cured coats of emulsion-type compositions, one common approach has been to include a surfactant. However, although this demonstrably lowers the surface tension of water as the to dispersion medium, the wettability of the film substrate surface leaves something to be desired. Also, surfactant remaining within the cured coat often has a poor compatibility with silicone, sometimes clouding the applied coat and causing appearance defects to arise. One solution has been to use a water-soluble thickener or thixotropic agent to suppress crawling by emulsion-type composition coats on the surface of film substrates. However, as with surfactants, this approach has not been effective for improving the appearance.
One way to improve the silicone phase within emulsion-type compositions is emulsion particle size control, but this is not a practical solution for resolving the problem of smoothness. A smaller particle size and narrower particle size variance results in a silicone coat having a smoother surface state following vaporization of the aqueous phase, but as the particle size becomes smaller under a limited surfactant concentration, the stability of the emulsion particles themselves decreases and agglomeration and coalescence tend to proceed; ultimately, the particle size variance broadens and the particles become larger in size, and so a smooth coated surface state is not obtained. Harsh conditions that most compromise the stability of the emulsion particles are curing steps that bring about a higher concentration by means of a rapid rise in temperature or water vaporization. Because the temperature rise conditions at in-line coating in particular are set so as to accommodate film processing, this cannot help but be exceedingly disadvantageous for emulsion-type compositions. Even when the amount of surfactant is increased and the particle size is reduced so as to ensure stability of the emulsion particles, clouding of the applied coat becomes very bad, in addition to which the release properties end up worsening due to a decrease in curability. Therefore, particularly in the production of release films, improvement by way of emulsion particle size control is currently an ineffective approach.
An additional problem is the decline in appearance and surface smoothness of the release film as the curing reaction proceeds over time (pot life) following preparation of the release composition. Normally, the curing reaction within the release composition can be kept from proceeding by the suitable addition of a regulator. However, in the case of emulsion-type compositions, as the curing reaction proceeds over time (pot life), the physical stability of the emulsion particles is also greatly affected. This difference influences the process of, in the curing step, the agglomeration and coalescence of emulsion particles to form a uniform silicone layer, and appears as a difference in the surface state and transparency of the final cured coat. At in-line coating in particular, the temperature of the emulsion composition prior to coating which is positioned and circulates in close proximity to the high-temperature film processing line tends to rise and the coating time also increases, further aggravating the effects. Accordingly, there exists a desire for an approach that differs from the use of a regulator.
The relationship between the physical stability of emulsion particles and their volume-average particle size has hitherto been pointed out. For example, JP No. 4823403 (Patent Document 1) describes setting the volume-average particle size of a platinum-alkenyl siloxane complex catalyst for an emulsion-type hydrosilylation reaction to 1 μm or less. However, changing the size of the emulsion particles was not observed to be effective whatsoever in satisfying the performance that is the object of the subsequently described instant invention.
In emulsion-type compositions, prior to the curing step, it is desired that addition reactions be suppressed as much as possible while keeping the emulsion state stable. In the curing step, it is desired that the process of change in the emulsion particle size itself, wherein the size of the emulsion particles rapidly increases toward formation of a continuous silicone phase, be controlled so as to reproducibly form a smooth, transparent cured coat. The emulsion particle size is most likely an effective indicator for maintaining and stabilizing a state in which the particle size itself does not change. However, in cases where it does change, the presumption is that the particle size cannot serve as the leading indicator.
Hence, there are limitations to the improvement effects achieved by existing methods of improvement, and so there exists a desire for an emulsion-type composition which can be employed in the production of release paper or release film having an appearance/transparency and surface smoothness that can stand up to use in a broad range of applications.